1. Field of the Invention
The present invention relates to a negative photoresist composition for use in the formation of thick films, as well as to a photoresist film and a method of forming bumps using the photoresist film. Specifically, the invention relates to an alkali-developable negative photoresist composition for the formation of thick films which is suitable for photofabrication such as bump formation, wiring, interlayer insulating film formation, circuit protective film formation and processing and manufacture of precision parts, carried out when circuit substrates are manufactured and semiconductors and electronic parts are packaged on the circuit substrates.
2. Description of the Related Art
The photofabrication is a generic term for techniques in which a photosensitive resin composition is coated on the surfaces of process articles and the coating films formed are patterned by photolithography, followed by chemical etching or electrolytic etching using the patterns as masks, or electroforming chiefly using electroplating, any of which are applied alone or in combination, to fabricate various precision parts. This is prevalent in the current high-precision microfabrication techniques.
With the downsizing of electronic equipment, there are a rapid progress toward higher integration of LSIs and toward ASICs (application specific integrated circuits), and a demand for multipin thin-film packaging for mounting LSIs on electronic equipment, where the bare chip packaging carried out by the TAB system or flip-chip system, has attracted notice. In such multipin packaging, protruded electrodes of 20 μm or more in height, called bumps serving as connecting terminals, must be highly precisely arranged on the substrate, and it has become more required to make the bumps higher in precision so as to be adaptable to any further miniaturization of LSIs in future. In addition to the formation of the connecting terminals, a rewiring process is performed to form wiring between the chip and the connecting terminals in many cases. In this procedure, the wiring is patterned with the use of a thick resist film about 5 to about 20 μm in thickness.
Photoresists for the formation of thick films are used as materials for the formation of such bumps or for rewiring. The term “photoresist for the formation of thick films” (hereinafter referred to as “thick-film photoresist”) used herein means and includes resists that can form films having a thickness of at least about 5 μm on substrates. Using such a patterned thick film as a mask, bumps are formed by plating process.
For example, Japanese Patent Laid-Open Nos. 10-207057, 2000-39709 and 2000-66386 disclose thick-film photosensitive resin compositions which are used for the formation of bumps or for wiring. These conventional thick-film photosensitive resin compositions require large amounts of reaction initiators in order to sufficiently react overall of the resulting resist films each having a large thickness. However, large amounts of reaction initiators may deteriorate compatibility or stability in preservation. Demands have therefore been made on reaction initiators having higher sensitivity.
Chemically amplified resists containing acid generators have been used as photosensitive resist compositions having high sensitivity. In these chemically amplified resists, a protonic acid is generated from the constitutive acid generator upon irradiation of active light or radiant ray and then induces an acid catalytic reaction with a base resin in the resin composition due to heat treatment after exposure. Thus, the chemically amplified resists have significantly higher sensitivity than conventional resists each having a photoreaction efficiency (a reaction per photon) of less than 1. As an example of chemically amplified negative resists, L. E. Bogan et al. disclose a resist using polyvinylphenol and a melamine derivative in combination in Proceeding of SPIE, 1086, 34-47 (1989). However, when thick films are formed using these chemically amplified resists, the resulting thick films invite cracking and do not exhibit required plating resistance.